Fuel cell flow field having metal bipolar plates

ABSTRACT

A bipolar plate ( 30, 30 ′) for use in a fuel cell ( 12, 14 ) includes a first metal layer ( 40   a ) having a first corrosion potential and a second metal layer ( 40   b ) that tends to grow an oxide layer ( 42, 42 ′) during operation of the fuel cell ( 12, 14 ). The second metal layer ( 40   b ) includes a second corrosion potential such that there is a corrosion potential gradient between the first metal layer ( 40   a ) and the second metal layer ( 40   b ) that resists growth of the oxide layer ( 42, 42 ′).

FIELD OF THE DISCLOSURE

This disclosure generally relates to fuel cells and, more particularly,to flow field plates for fuel cells.

DESCRIPTION OF THE RELATED ART

Fuel cells are widely known and used for generating electricity in avariety of applications. Typically, a fuel cell unit includes an anodeelectrode, a cathode electrode, and an ion-conducting polymer exchangemembrane (PEM) between the anode electrode and the cathode electrode forgenerating electricity from a known electrochemical reaction. Severalfuel cell units are typically stacked together to provide a desiredamount of electrical output. Typically, a bipolar plate is used toseparate adjacent fuel cell units. In many fuel cell stack designs, thebipolar plate functions as a flow field to deliver reactant gases,remove waste heat, and to conduct electrons within an internal circuitas part of the electrochemical reaction to generate the electricity.

Presently, the bipolar plates are made of graphite to provide a desiredlevel of electrical conductivity. The graphite is also resistant tocorrosion within the relatively harsh environment of the fuel cell.However, a significant drawback of using graphite is that the plate mustbe relatively thick to achieve a desired strength, thereby reducingpower density of the fuel cell stack. Alternatively, the bipolar platesare made of a metal. However, many commonly used metals corrode in thefuel cell environment, thereby producing an electrically insulatinglayer that undesirably increases an electrical contact resistancebetween the bipolar plate and the electrodes and may poison theelectrodes to limit the lifetime of the fuel cell. Therefore, arelatively thin bipolar plate that resists corrosion is needed toincrease the volumetric power density and reduce the expense of a fuelcell stack.

SUMMARY OF THE DISCLOSURE

One example bipolar plate for use in a fuel cell includes a first metallayer having a first corrosion potential and a second metal layer havinga tendency to grow an electrically passive layer during operation of thefuel cell. The second metal layer includes a second, different corrosionpotential such that there is a corrosion potential gradient, ordifference, between the first metal layer and the second metal layerthat is operative to control growth of the electrically passive layer.

An example method for use with a fuel cell includes the steps of forminga bipolar plate using a metal layer having a tendency to grow anelectrically passive layer and establishing a corrosion potentialgradient for controlling a nominal growth rate of any electricallypassive layer growing at the metal layer.

In another aspect, a fuel cell assembly includes a cell stack having oneor more electrodes and one or more bipolar plates associated with theelectrodes. Each of the bipolar plates includes a first metal layerhaving a first corrosion potential and a second metal layer having atendency to grow an electrically passive layer during operation of thefuel cell. The second metal layer includes a second, different corrosionpotential such that there is a corrosion potential gradient between thefirst metal layer and the second metal layer that is operative tocontrol growth of any electrically passive layer at the second metallayer.

The various features and advantages of this disclosure will becomeapparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates selected portions of an example fuel cell stack.

FIG. 2 illustrates an example bipolar plate according to the sectionline shown in FIG. 1.

FIG. 3 illustrates an example bipolar plate having a metal layer mesh.

FIG. 4 illustrates an example bipolar plate having a third metal layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates selected portions of an example fuelcell stack 10 for generating electricity. In this simplified example,the fuel cell stack 10 includes fuel cells 12 and 14 that each has acathode 16 that receives a first reactant gas and an anode 18 thatreceives a second reactant gas to generate an electric current using aknown reaction. Each fuel cell 12 and 14 includes a polymer exchangemembrane (PEM) 20 that separates a cathode catalyst 22 from an anodecatalyst 24. Gas diffusion layers 28 distribute the reactant gases overthe respective cathode catalyst 22 and anode catalyst 24 in a knownmanner, and a metal bipolar plate 30 separates the fuel cells 12 and 14.

FIG. 2 schematically illustrates the portion of the example fuel cell 12according to the section shown in FIG. 1. In this example, the metalbipolar plate 30 includes a first metal layer 40 a galvanically coupledwith a second metal layer 40 b. In one example, the metal layers 40 aand 40 b are in direct contact with each other such that the contactprevents significant amounts of water and/or reactant gas from comingbetween the metal layers 40 a and 40 b at the contact point(region/area). Furthermore, the metal layers 40 a and 40 b are exposedto the same reactant gas environment in the fuel cell stack 10 duringoperation that produces a galvanic current between the metal layers 40 aand 40 b. In one example, the metal layers 40 a and 40 b aremetallurgically bonded using welding, diffusion bonding, highcompression force, etc. or other method for achieving intimate contact.

The first metal layer 40 a has a first corrosion potential and thesecond metal layer 40 b has a second, different corrosion potential suchthat there is a corrosion potential gradient between the first metallayer 40 a and the second metal layer 40 b. In one example, corrosionpotential is determined from a known galvanic series or from corrosionpotential evaluations in a simulated fuel cell environment.

In operation, the electrochemical reactions within the fuel cells 12 and14 produce a relatively harsh environment for the metal bipolar plate30. For example, the cathode 16 produces an acidic, oxidizingenvironment and the anode 18 produces an acidic, reducing environment.In the disclosed example, the harsh environment at the cathode tends togrow an oxide layer 42 at the metal bipolar plate 30. In one example,the oxide layer 42 is a metal oxide of a metal used in the metal bipolarplate 30, such as chromium oxide or iron oxide or mixtures thereof. Theoxide layer 42 is generally a poor electrical conductor and increaseselectrical contact resistance of the metal bipolar plate 30 (i.e., theability of the metal bipolar plate 30 to conduct electrons from thecathode 16 or the anode 18). Thus, in this example the oxide layer 42 ispassive and protects the underlying second metal layer 40 b fromcorrosion.

In the disclosed example, the first metal layer 40 a and the secondmetal layer 40 b cooperate to resist growth of the electrically passivelayer 42 to maintain a desired thickness of the oxide layer 42.Resisting growth provides the benefit of maintaining a desirable levelof electrical contact resistance of the metal bipolar plate 30.

In the illustrated example, the second metal layer 40 b has a morenegative corrosion potential than the first metal layer 40 a (i.e., thefirst metal layer 40 a is more noble). The difference in corrosionpotential (i.e., the corrosion potential gradient) produces a corrosioncurrent 44 of electrons from the second metal layer 40 b to the firstmetal layer 40 a. In this example, the flow 44 results in dissolution ofthe oxide layer 42 to maintain or reduce a thickness (t) of the oxidelayer 42. In some examples, the thickness (t) is maintained at a desiredthickness (t) suitable to protect the underlying layer 40 b fromcorroding. Alternatively, the first metal layer 40 a is more negativethan the second metal layer 40 b. It is believed that this would induceoxygen reduction reactions that control the thickness (t) of the oxidelayer 42.

In one example, the oxide layer 42 is a metal oxide of a metal of thesecond metal layer 40 b. The flow 44 of electrons reduces the metaloxide, resulting in a thinner passive film of the passive layer. Thethinner passive film provides the benefit of better electricalconductivity. Maintaining the desirable thickness (t), in turn providesenhanced, that is reduced, long term electrical contact resistance ofthe metal bipolar plate 30.

In another example, the second corrosion potential is about 200 mVdifferent than the first corrosion potential. In one example, thedifference is about 150 mV. In another example, the second corrosionpotential is between about 30 mV and about 50 mV less than the firstcorrosion potential (i.e., more negative). Such a difference providesthe benefit of a flow 44 of electrons that provides a desirable rate ofdissolution of the oxide layer 42 without significant dissolution of thebase metal of the second metal layer 40 b or poisoning of the fuel cellcatalyst 22, 24. Differences in corrosion potential that are above 200mV may result in a relatively large galvanic current that may result indissolution of the base metal into the gas diffusion layer 28, where themetal can contact and poison the fuel cell catalyst 22, 24. However, ifthe difference is below 30 mV, the rate of dissolution may not be enoughto significantly control or reduce the size of the oxide layer 42. Giventhis description, one of ordinary skill in the art will recognizesuitable differences in corrosion potential to meet the needs of theirparticular design.

In some examples, the corrosion potential gradient functions to controlthe growth rate of the oxide layer 42 as described above such that thethickness (t) does not exceed a predetermined threshold thickness. Insome examples, the corrosion potential gradient functions primarily whenthe fuel cell stack 10 is inactive (e.g., when reactant gas supply isshut off) to reduce the thickness (t) of the oxide layer 42 during fuelcell inactivity.

In the disclosed example, the first metal layer 40 a is made of a firsttype of metal (or metal alloy) and the second metal layer 40 b is madeof a second, different type of metal (or metal alloy). In one example,the first type of metal is a stainless steel and the second type ofmetal is a nickel alloy or nickel-chromium alloy, which provide adesirable corrosion potential gradient for some situations.

In one example, the first metal layer 40 a has a nominal composition ofabout 50 wt % to 70 wt % of Fe, about 9 wt % to 26 wt % of Ni, about 12wt % to 25 wt % of Cr, about 2 wt % to 4 wt % of Mo, and about 1 wt % to2 wt % of Mn. In some examples, the composition includes less than 3 wt% of other common impurity elements, such as P, S, Si, and C. In afurther example, the amount of Fe is about 60 wt % to 65 wt %, theamount of Ni is about 10 wt % to 14 wt %, and the amount of Cr is about16 wt % to 18 wt %.

In this example, the second metal layer 40 b has a nominal compositionof about 55 wt % to 75 wt % Ni, about 15 wt % to 23 wt % Cr, about 2 wt% to 25% of Mo, about 10 wt % to 14 wt % W, about 2 wt % to 5 wt % ofFe, and about 0.5 wt % to 1 wt % of Mn. In some examples, thecomposition includes less than 1 wt % of other elements, such as Al, B,La, Si, and C.

The above nominal compositions provide the benefit of a desirablecorrosion potential gradient between the first metal layer 40 a and thesecond metal layer 40 b. The term “about” as used in this descriptionrelative to the compositions refers to possible variation in thecompositional percentages, such as normally accepted variations ortolerances in the art.

In this example, at least one of the layers 40 a or 40 b isnon-continuous. FIG. 3 illustrates an example in which the second metallayer 40 b comprises a mesh 56 having openings 58. The mesh 56 providesthe benefit of permitting control over a ratio of exposed surface areaof the first metal layer 40 a and the second metal layer 40 b. Forexample, providing smaller openings 58 decreases the exposed area of thefirst metal layer 40 a. In contrast, providing larger openings 58increases the exposed area of the first metal layer 40 a. The mesh 56can be selected to achieve a desired ratio of exposed surfaces areaswhich corresponds to the galvanic current supported by each layer andthe ability to control the thickness (t) of the oxide layer 42.

Controlling or selecting the ratio of exposed surface area provides thebenefit of controlling the galvanic current within the metal bipolarplate 30 to thereby control the rate of dissolution, or in somecircumstances the growth rate, of the oxide layer 42. In one example,the contact area is used in combination with known corrosion potentialsof the first metal layer 40 a and the second metal layer 40 b to producea desirable dissolution rate of the oxide layer 42. Given thisdescription, one of ordinary skill in the art will recognize alternativenon-continuous patterns to meet their particular needs.

FIG. 4 illustrates a typical complete cell version of a metal bipolarplate 30′. In this example, the metal bipolar plate 30′ is substantiallysimilar to the metal bipolar plate 30 shown in FIG. 2, except that themetal bipolar plate 30′ includes a third metal layer 40 c galvanicallycoupled to the first metal layer 40 a. In this example, the first metallayer 40 a is between the second metal layer 40 b and the third metallayer 40 c.

The example third metal layer 40 c has a nominal composition that isequal to the nominal composition of the second metal layer 40 b, such asa composition described above. As with the second metal layer 40 b (inthis example considered the cathode environment), the third metal layer40 c includes a corrosion potential that is established by the reactantgas environment on that side of the cell, typically the anode gasenvironment. Thus, the third metal layer 40 c functions in a similarmanner as the second metal layer 40 b.

The disclosed example metal bipolar plates provide the benefit ofimproved volumetric power density compared to previously known graphitebipolar plates. The example metal bipolar plates resist growth of theoxide layer 42, 42′ to thereby allow the use of metallic materials inthe relatively harsh environment of a fuel cell stack withoutsignificant penalty to conductivity. Moreover, the high strength ofmetallic materials compared to graphite allows the example bipolarplates to be relatively thinner compared to graphite plates. Thinnerbipolar plates reduce the cell stack assembly size and provide morepower per volume of a fuel cell stack.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthe essence of this disclosure. The scope of legal protection given tothis disclosure can only be determined by studying the following claims.

1. An article for use in a fuel cell, comprising: a bipolar plate having a first metal layer having a first corrosion potential; and a second metal layer galvanically coupled with the first metal layer and having an oxide layer during operation of the fuel cell, the second metal layer having a second, different corrosion potential such that there is a corrosion potential gradient between the first metal layer and the second metal layer that is operative to control growth of the oxide layer at the second metal layer.
 2. The article as recited in claim 1, wherein the second corrosion potential is between about 200 mV different relative to the first corrosion potential.
 3. The article as recited in claim 2, wherein the second corrosion potential is between about 30 mV and about 50 mV different relative to the first corrosion potential.
 4. The article as recited in claim 1, wherein the oxide layer has an associated growth rate, and wherein the corrosion potential gradient reduces the growth rate.
 5. The article as recited in claim 1, wherein the first metal layer includes a stainless steel and the second metal layer includes at least one of a nickel-based alloy or a nickel-chromium based alloy.
 6. The article as recited in claim 5, wherein: the first metal layer includes a nominal composition of between about 50 wt % to 70 wt % of Fe, about 9 wt % to 26 wt % of Ni, about 12 wt % to 25 wt % of Cr, about 2 wt % to 4 wt % of Mo, and about 1 wt % to 2 wt % of Mn; and the second metal layer includes a nominal composition of about 55 wt % to 75 wt % Ni, about 15 wt % to 23 wt % Cr, about 2 wt % to 25% of Mo, about 10 wt % to 14 wt % W, about 2 wt % to 5 wt % of Fe, and about 0.5 wt % to 1 wt % of Mn.
 7. The article as recited in claim 6, including a third metal layer having a nominal composition equal to the nominal composition of the second metal layer, wherein the first metal layer is between the second metal layer and the third metal layer.
 8. The article as recited in claim 1, wherein the first metal layer includes a solid, continuous planar section and the second metal layer includes a non-continuous section directly adjacent the solid, continuous planar section.
 9. The article as recited in claim 1, wherein the non-continuous section comprises a mesh.
 10. The article as recited in claim 6, wherein the amount of Fe is about 60 wt % to 65 wt %, the amount of Ni is about 10 wt % to 14 wt %, and the amount of Cr is about 16 wt % to 18 wt % in the nominal composition of the first metal layer.
 11. The article as recited in claim 1, wherein corrosion potential gradient is operative to prevent a thickness of the oxide layer from exceeding a threshold.
 12. The article as recited in claim 1, wherein the corrosion potential gradient is operative to reduce a thickness of the oxide layer.
 13. The article as recited in claim 1, wherein the corrosion potential gradient is operative to prevent an electrical contact resistance from exceeding a threshold.
 14. A method for use with a fuel cell, comprising: (a) forming a bipolar plate using a metal layer having a potential to grow an oxide layer; and (b) establishing a corrosion potential gradient for controlling a nominal growth rate of the oxide layer growing at the metal layer.
 15. The method as recited in claim 14, including galvanically coupling another metal layer of the bipolar plate to the metal layer of the bipolar plate.
 16. The method as recited in claim 15, including controlling the nominal growth rate to maintain a selected conductivity of the oxide layer.
 17. The method as recited in claim 16, including galvanically dissolving the metal within the oxide layer to reduce the growth rate.
 18. The method as recited in claim 15, including controlling a rate of galvanic dissolution by selecting a desired ratio of exposed surface area between the metal layer and the another metal layer.
 19. The method as recited in claim 14, including permitting growth of the oxide layer while operating the fuel cell to generate an electric current, and galvanically dissolving the oxide layer when the fuel cell is not operating to generate an electric current.
 20. A fuel cell assembly comprising: a cell stack having a plurality of electrodes; and a plurality of bipolar plates associated with corresponding electrodes, each of the bipolar plates comprising: a first metal layer having a first corrosion potential; and a second metal layer galvanically coupled with the first metal layer and having a potential to grow an oxide layer during operation of the fuel cell, the second metal layer having a second, different corrosion potential such that there is a corrosion potential gradient between the first metal layer and the second metal layer that is operative to control growth of the oxide layer at the second metal layer.
 21. The assembly as recited in claim 20, wherein the first metal layer and the second metal layer are in direct contact. 